1. Field of the Invention
The present invention relates to an information recording method, a recording condition detection method, and an information recording apparatus.
2. Description of the Related Art
A recording condition of an optical disk is represented by two parameters, namely, “asymmetry” and “β”.
Each of “asymmetry” and “β” represents an asymmetric characteristic of the waveform of a reproduction signal (RF (Radio Frequency) waveform) which is obtained when information recorded on an optical disk is reproduced.
“Asymmetry” is derived from a reproduction signal obtained by DC (direct-current) coupling, whereas “β” is derived from a reproduction signal obtained by AC (alternating-current) coupling.
A recording apparatus for recording information on an optical disk changes the power of a laser light irradiated on the optical disk in accordance with a digital signal obtained by EFM (Eight to Fourteen Modulation). Due to this, a plurality of pits representing information to be recorded are formed on the optical disk.
The time width (pulse width) of the high level and the low level of the aforementioned digital signal is generally regulated to three times to eleven times as large as a predetermined reference time width T.
Since the recording apparatus changes the power of a laser light in accordance with pulse widths 3T to 11T, the lengths of pits to be formed also vary in accordance with the pulse widths 3T to 11T.
FIGS. 1A and 1B respectively show waveforms of reproduction signals obtained from pits having lengths corresponding to the aforementioned pulse widths 3T to 11T. Specifically, FIG. 1A is a diagram showing waveforms of reproduction signals (HF (High Frequency) signals) obtained by DC coupling, and FIG. 1B is a diagram showing waveforms of reproduction signals (HF signals) obtained by AC coupling.
As shown in FIGS. 1A and 1B, the frequencies and amplitudes of the reproduction signals vary in accordance with the lengths of the pits. For example, the amplitude of a reproduction signal obtained from a pit having the length of 3T (the shortest pit) is the smallest, while the amplitude of a reproduction signal obtained from a pit having the length of 11T (the longest pit) is the largest.
Let it be assumed in FIG. 1A that the local maximum level of the reproduction signal of 3T is I3L and the local minimum level thereof is I3P, and the local maximum level of the reproduction signal of 11T is I11L and the local minimum level thereof is I11P. In this case, the “asymmetry” is represented by the equation (1) below.Asymmetry={(I3L+I3P)/2−(I11L+I11P)/2}/(I11L−I11P)  (1)
Further, let it be assumed in FIG. 1B that AC-GND is zero level, the maximum level (land level) of the waveform envelopes is A1, and the minimum level (pit level) of the waveform envelopes is A2. In this case, “β” is represented by the equation (2) below.β=(A1+A2)/(A1−A2)  (2)
There is a relationship shown in FIG. 2 between the parameter β (asymmetry) and jitter, for example.
As shown in FIG. 2, when the value of β is extremely large or extremely small, the jitter value becomes large.
The value range of β in which jitter values are equal to or smaller than a tolerable level is generally called “power margin”. The power margin varies in accordance with the kinds of optical disks.
In a case where information is to be recorded on an optical disk having a narrow power margin, the information must be recorded such that the values of β become uniform.
When recording information, experimental writing is first performed in a PCA (Power Calibration Area) prepared on the optical disk, in order to perform so-called OPC (Optimum Power Calibration). As a result, the power of a laser light that realizes the optimum recording condition (optimum power) is derived.
Afterwards, the information to be recorded is recorded on the data recording area prepared on the optical disk, by the optimum power derived by the OPC.
However, although the information is recorded by the optimum power derived by the OPC, there are some cases where the recording condition can not be kept optimum, due to various causes described below.
Such causes are, for example:
(a) changes in the property inside the optical disk;
(b) changes in the angle between the optical axis of the laser light and the recording surface of the optical disk, caused by radial skews, warping of the disk, etc.;
(c) changes in the property of the optical disk, due to changes in the temperature between the time of performing the OPC and the time of recording information; and
(d) changes in the wavelength of the laser light, due to changes in the characteristic of a semiconductor element which emits the laser light, caused by changes in the temperature between the time of performing the OPC and the time of recording information.
As one method for solving the above described problems, there is a method called “running OPC”. The running OPC is for keeping the optimum recording condition, by detecting the recording condition even while information is recorded and by correcting the power of the laser light irradiated on the optical disk based on the detected recording condition.
Such a method for detecting the recording condition during information recording is disclosed in, for example, Unexamined Japanese Patent Application KOKAI Publication No. 2000-215454 (Publication 1), Unexamined Japanese Patent Application KOKAI Publication No. H9-270128 (Publication 2), and Unexamined Japanese Patent Application KOKAI Publication No. H9-91705 (Publication 3).
The technique disclosed in Publication 1 derives the recording condition during information recording, using the equation (3) below.Recording condition=(B value)/(PEAK value)  (3)                B value: the minimum intensity of a reflection light from an area where a pit is to be formed during information recording        PEAK value: the maximum intensity of a reflection light from the area where a pit is to be formed during information recording        
The technique disclosed in Publication 2 derives the recording condition during information recording, using the equation (4) below.Recording condition=Am/N−Bm  (4)                Am: the average of the maximum intensities of reflection lights from the areas where pits are to be formed        N: a constant corresponding to the kind of the optical disk        Bm: the average of the intensities of reflection lights which are observed when the reference time width T passes from the points from which the respective pits are to be formed        
The technique disclosed in Publication 3 derives the recording condition during information recording, using the equation (5) below.Recording condition=Vp/P1  (5)                Vp: the level of a reflection light from the area where a pit is to be formed during information recording        P1: the power of the laser light for forming pits        
However, the techniques disclosed in Publications 1, 2, and 3 are not able to keep the optimum recording condition derived by the OPC throughout the information recording in some cases.
Specifically, when the kinds of the optical disks are changed or operational environments of the recording apparatus are changed, the recording condition may not be represented correctly by the equations (3), (4), and (5). Therefore, the recording condition might be changed even when the running OPC is performed by using the equations (3), (4), and (5).
The techniques disclosed in Publications 1, 2, and 3 do not at all take into consideration the fact that the most appropriate calculation required to derive the recording condition is different depending on the kinds of the optical disks and the operational environments of the recording apparatus. Therefore, there are some cases where the techniques disclosed in Publications 1, 2, and 3 can not keep the optimum recording condition derived by the OPC throughout the information recording.
The contents of Publications 1, 2, and 3 described above are incorporated herein by reference.